In satellites the power amplifiers used are primarily traveling-wave tube amplifiers (TWTA, Traveling-Wave Tube Amplifier), which are generally designed as traveling-wave tube modules. These comprise a traveling-wave tube, which primarily determines the high-frequency characteristics, and a power supply, which generates the supply voltages for the traveling-wave tubes; in addition, they can also comprise a telemetry and/or telecommand interface to the satellite, and a control. The traveling-wave tube module can be complemented with a preamplifier (also called a channel amplifier), which can also contain a linearizer. In this combination, the device is known as a high-frequency power module.
A traveling-wave tube amplifies a high-frequency signal by passing an electron beam by a conductor, usually helical in shape, through which the high-frequency signal flows. If the conductor and the electron beam are configured appropriately, energy can be transferred from the electron beam to the high-frequency signal.
In general, a traveling-wave tube is designed to be operated at or near saturation. To achieve the greatest possible efficiency, multi-stage collectors, among other things, are used with a plurality of collector elements or collector stages that recover the majority of the residual energy from the consumed electron beam. The goal here is to select the voltages of the collector stages such that the traveling-wave tube achieves the greatest possible efficiency at saturation. When a high-frequency input signal is applied, the cathode current is distributed more or less uniformly to the various collector stages, thus resulting in a distribution of the power loss to all of the collector stages.
During idle operation, when the traveling-wave tube is operated without control, the electron beam does not give off any energy and therefore fully strikes the last collector stage. As a result, all of the power loss is concentrated on the last stage of the collector. In order to keep the power loss at an acceptable level in this case, the voltage is not freely selectable at this stage and, in certain circumstances, must be set at a lower value that would be necessary for optimum efficiency. This stage is therefore sometimes operated near its thermal capacity.
Consideration is being given to adding an additional collector stage that makes it possible to operate the last stage at a lower voltage, and thus with less power loss when it is not being controlled, but without a drop in efficiency. However, this additional stage makes both the power supply and the traveling-wave tube more complex and hence heavier and more expensive.